1. Field of the Invention
Many optical techniques require compact tunable laser sources of psec order optical pulses. Ideally, the pulse widths should be at the detection limit of the photodetectors, which is about 1-5 psec with current technology. Lasers based on single mode rare-earth-doped fibers are attractive candidates for such sources due to their intrinsic low volume and pump power requirements as disclosed in S. B. Poole, D. N. Payne and M. E. Fermann, Electron. Lett., 21,737 (1985). The aim of the present invention is the attainment of high output power optical pulses from such lasers through dispersive control of the pulse width.
2. Discussion of the Prior Art
Compact passively modelocked fiber lasers capable of producing pulses of shorter than 100 fsec with energies approaching 1 nJ are now well known in the art, as disclosed in M. E. Fermann, M. Hofer, F. Haberl, A. J. Schmidt and L. Turi, Opt. Lett., 16, 244 (1991). However, when the pulse widths are increased to the psec level, the pulse energies decline to (typically) just a few tens of pJ. This drop-off is mainly due to limitations set by the stability requirements for the various laser designs typically employed.
For example, bandwidth-limited psec pulses can easily be produced by actively modelocked fiber lasers; however, laser stability then requires that the non-linearity of the cavity remain low (i.e., the soliton period remain long compared to the cavity length); to this end, a nonlinear phase delay of &lt;0.1 .pi. should be ensured, as disclosed by S. M. J. Kelly, K. Smith, K. J. Blow and N. J. Doran, in Opt. Lett., 16, 1337 (1991). In conjunction with the large induced intra-cavity loss from standard integrated modulators and the high repetition rates typically employed, this leads to small generated pulse powers.
On the other hand, in conventional passively modelocked fiber lasers, psec pulses are generated by providing a large amount of negative dispersion (solition supporting dispersion)inside the cavity, which is achieved by using long lengths (&gt;100 m) of optical fiber inside the cavity, as disclosed by D. Taverner, D. J. Richardson and D. N. Payne, in Opt. Soc. Am. Topical Meeting on Nonlinear Guided Wave Phenomena, Cambridge, 1993, Opt. Soc. Am. Techn. Dig. Series, 15, 367 (1993). Clearly, the possible pulse energies are then limited by the resulting large non-linearity of the cavity; in addition, the stability of such long lasers deteriorates due to the long fiber lengths employed.
Recently, bandwidth-limited, high power psec-order (2 nJ, 5 psec) pulses have been obtained from a fiber laser passively modelocked with a saturable absorber as disclosed by Barnett et al., in Opt. Soc. Am. Techn. Dig. Ser., 8, 52 (1994), which is hereby incorporated by reference herein. However, to date, questions still remain about the long-term reliability of such systems due to the susceptibility of erbium fiber lasers to Q-switching, and the low damage thresholds of typical saturable absorber materials. Further, due to the resonant nature of the employed nonlinearity for modelocking, such systems only allow for very limited tunability and require a very good match between saturable absorber and laser gain medium. Equally, there exists no simple scaling laws for such lasers, which creates great difficulties in laser design and optimization of laser performance for a given application.
Low-power psec-pulses have also been obtained by incorporating un-chirped fiber Bragg gratings into a fiber laser cavity, as described by D. U. Noske et al., Optics Communications 108 (1994) 297-301. However, due to the non-uniform dispersion profile of such gratings, the pulses are typically not bandwidth-limited and only have a very limited turning range.